ESEMs are distinguished from high-vacuum SEMs in that they are capable of viewing samples maintained at pressures greater than in the order of 100 Pa.
It is known from U.S. Pat. No. 5,250,808 to adapt a conventional scanning electron microscope (SEM) to scan samples maintained at an elevated pressure, for example at an atmospheric pressure of substantially 1000 mBar, 760 Torr, 101.3 kPa. Such an adapted SEM comprises an electron gun for generating an electron beam, one or more electromagnetic lenses with associated electron beam deflectors for demagnifying and scanning the electron beam, a sample chamber for housing a sample to be viewed by scanning the demagnified electron beam thereover, a vacuum pumping system for evacuating the apparatus to generate a vacuum therein, one or more electron sensing assemblies for detecting secondary and backscattered electrons emitted from the sample in response to bombardment thereof by the scanned demagnified electron beam, and an electronic control system for controlling the SEM, the control system including one or more image displays. The adapted SEM additionally comprises in the electromagnetic lens nearest the chamber, namely the objective lens, a series of differentially pumped diaphragms. Each diaphragm includes therein an aperture through which the electron beam can pass. The diaphragms are designed to be installed permanently into the adapted SEM. The diaphragms define at least two interior passages which are in communication with vacuum pumping ports of the SEM. Above all, the adapted SEM is not designed to function with its series of differentially pumped diaphragms removed.
Inclusion of the diaphragms and maintaining the sample at elevated pressure introduces problems into operation of the adapted SEM.
Such problems result, for example, from electron scattering in higher-pressure regions surrounding the sample resulting in electron probe blurring. Moreover, inclusion of differentially pumped diaphragms imposes limitations of longer objective lens working distances and hence increased electron probe spherical aberration. Furthermore, in extreme cases, differentially pumped diaphragms can be an electron beam semiangle-limiting actor which can render microscope optical alignment problematic and reduce available electron probe current.